Variable inductors can be used in many circuit applications, such as resonant circuits which vary the inductance of circuit elements to vary the resonant frequency of the circuit. An example of a resonant circuit system is described in United States Patent Publication 2002/0121285, the entire teachings of which are herein incorporated by reference.
The simplest way to obtain a variable inductor is by mechanical movement of a connector along an inductive element. However, mechanical movement lacks the response time required for real time control. Further, mechanical movement-type variable inductors have a tendency to lock-up magnetically. Therefore, variable inductors have been designed to vary the inductance of a circuit element by means of an electrical signal rather than by mechanical movement.
The saturation effect of magnetic materials can be employed to create a current controlled variable inductor. These type of variable inductors typically have a limited variation range of 1 to 10 and suffer from parasitic effects such as capacitance and voltage across each control winding that limit the quality (Q) factor of the inductor. Additionally, such current controlled variable inductors require very high control currents in the range of 0 to 500 mA.
The inductance of an inductive circuit element is related to the permeability of the magnetic core and the number of turns:
                              L          =                                    μ              o                        ⁢                          N              2                        ⁢                          A              l                                      ;                            equation        ⁢                                  ⁢        1            
where L is the inductance of an inductive circuit element;
μo is the permeability of the magnetic core;
A is the cross-sectional area of the magnetic core;
N is the number of turns of the inductive element; and
l is the length of the inductive element.
FIG. 1 illustrates a current controlled variable inductor 10 in which the inductance L20 of main winding 20 is controlled by the current (Ic) delivered to outer control windings 22 and 24. Since the center leg 34 is not saturated, the minimum inductance L20 is limited by the number of turns (N) and the magnetic permeability of the core material of the center leg 34. The voltage across each control winding 22 and 24 and the parasitic capacitances of control windings 22 and 24 limit the winding ratio and/or the operating frequency. The inductance of the control windings 22 and 24 changes substantially with the control current (Ic).
A magnetic core 30 is shown consisting of a magnetic material which can be saturated, with three legs 32, 34 and 36. The outer legs 32 and 36 have identical control windings 22 and 24 that are connected in series. The magnetic path for main winding 20 includes outer legs 32 and 36, center leg 34 and the connecting portions 40, 42, 44, and 46. If the control current (Ic) through control windings 22 and 24 becomes large enough to saturate the outer legs 32 and 36 of the core 30, the inductance L20 of main winding 20 decreases because a portion of the magnetic path for the main winding 20 is saturated. The higher the control current (Ic) is made, the lower the inductance L20. However, the center leg 34 will not be saturated due to the control current (Ic). Control windings 22 and 24 are wound and connected such that the magnetic flux (Φc1, Φc2) in respective legs 32 and 36 of the core 30 arising from the control current (Ic) through the outer control windings 22 and 24 is equal and points in opposite directions. The opposing magnetic flux (Φc1, Φc2) results in cancellation in the center leg 34 of the core 30. The flux cancellation prevents coupling of AC signals between the main winding 20 and the control windings 22 and 24. AC voltage applied across the terminals of main winding 20 induces a voltage in both of the control windings 22 and 24.
The induced voltage is related to the magnetic flux Φc and the number of turns:
                                          e            ⁡                          (              t              )                                =                      N            ⁢                                          ⅆ                ϕ                                            ⅆ                t                                                    ;                            equation        ⁢                                  ⁢        2            
where e(t) is the induced voltage as a function of time;
Φ is the magnetic flux
      (                  ⅆ        ϕ                    ⅆ        t              )    ;and
N is the number of turns of the inductive element.
Although the voltages in the control windings 22 and 24 have opposite polarity such that the voltage across the series connection of control windings 22 and 24 have a net zero voltage, the voltage with respect to ground increases with each respective turn of the control windings 22 and 24. That is, the voltage at point B is greater than the voltage at point A.